1. Technical Field
The present disclosure relates to manufacturing bolus for use in radiotherapy and more specifically to customized, user-specific bolus for accurately targeting a specific treatment area. The disclosure also addresses creating bolus for different types of therapy, including photon therapy, electron therapy, and proton therapy. The disclosure also describes how a bolus can be incorporated into an immobilization device, and how a custom, 3D-printed bolus can incorporate dosimeter functionality.
2. Introduction
Radiotherapy is a treatment for disease in which an affected part of the body of a patient is exposed to ionizing radiation. For a range of treatment applications, an adequate surface dose is required, particularly in the presence of superficial target volumes. Since megavoltage radiation beams do not deposit maximal dose at the skin surface, in these cases surface dose can be increased by overlaying a tissue equivalent material, called bolus. Bolus is most commonly used in conjunction with electron therapy which is well suited to treatment of superficial lesions with a single beam. A second purpose of bolus is controlling the depth in tissue at which a therapeutic dose of radiation is deposited, and modulating this depth as a function of position across the beam.
Currently, radiation therapists manually create bolus. For example, a radiation therapist can apply wax or thermoplastic sheets to the patient surface. Often, a radiation therapist heats the wax or other material to make it more pliable or malleable. The radiation therapist can apply the bolus material in one or more layers to conform to the patient surface. Often the radiation therapist attempt to manually create a regular geometry or a flat surface at the location of beam incidence. The patient and radiation therapist must then wait while the bolus material cools.
This manual approach is limited in regard to accuracy, practicality and quality of the delivered treatment. First, this process is labor intensive because it involves manual application of bolus material. This occupies the patient, potentially multiple staff members, as well as clinic space, often in an expensive or valuable computed tomography (CT) suite. Second, the bolus should conform well to the patient skin, even in situations where the geometry is complex, such as an outer ear, canthus, lip, or other extremities. The capacity of manually produced bolus to conform to irregular surfaces is limited. Inaccuracy of bolus fabrication can result in air gaps between the bolus and patient surface. Air gaps, in turn, can result in substantial inaccuracies in delivered surface dose, for example, exceeding 10%. In practice, this sometimes prompts filling of air gaps with wet gauze, however the variability in the wetness of the gauze causes inconsistency in delivered dose. Third, bolus is commonly pre-defined in the planning system as a water equivalent, uniform layer on the patient surface. The similarity of the planned and fabricated bolus is limited with regard to both thickness and curvature, particularly in the presence of steep, complex or curved surfaces. This compromises the accuracy of the delivered dose distribution relative to the plan. Fourth, other than controlling the depth of penetration of an electron beam into tissue, manually manufactured bolus does not achieve conformity between the radiation dose and the target volume. Most commonly, the high dose region will encompass the deepest aspect of an irregularly shaped tumor but also a volume of surrounding healthy tissue which would be preferable to avoid exposing to excess radiation.